1. Field of the Invention
The present invention relates to a motor controller for use in a desk-top cutting machine using an endless belt as means for transmitting motor driving force.
2. Description of the Related Art
In the cutting work for a workpiece 32 as shown in FIG. 8, in order to obliquely cut both sides of the workpiece 32 without reversing the workpiece 32, a cutting blade must be made tiltable (45 degree or so) on both sides of the workpiece 32. In a conventional desk-top cutter as shown in FIG. 7, however, a motor 111 projects from its side in the direction substantially parallel to a cutting blade shaft 110 of a cutting blade section 108. For this reason, even if the cutting blade 108 is tilted towards a side to which the motor 111 projects, an end 117a of a motor housing 117 hits on an upper surface of a base 101 before the tilting angle reaches a predetermined angle, thus making it impossible to cut the workpiece 32.
Therefore, there has been proposed, for example, in Japanese Utility Model Unexamined Publication No. Hei 5-68601, a method of belt driving in which a motor is located above the cutting blade and an endless belt which is made of an elastic material such as rubber is extended between the motor shaft and the cutting blade shaft which are apart from each other vertically, in order to make the cutting blade tiltable on left and right sides.
In order that the cutting blade can be tilted by about 45 degree, the width of the cutting blade must be narrowed so that a space of the cutting blade is minimized. Usually, in the case of belt driving, the endless belt is given a width which permits the maximum torque of the motor to be transmitted. However, in a desk-top cutting machine, because of the above problem, the endless belt will be limited to the lowest width which permits transmission of the torque necessary to cut the workpiece.
Conventionally, in the above-mentioned width of the endless belt, because the transmission torque and rpm (revolutions per minute) of the endless belt are low, when they exceed their limitations, slippage occurs between the motor shaft and endless belt and between the cutting blade and endless belt. Particularly, slippage occurs when the acceleration torque in motor starting, the deceleration torque in abrupt motor stopping and load torque in overloading during cutting exceed the permissible transmission torque for the endless belt. Then, the friction between the motor shaft and endless belt and between the cutting blade shaft and endless belt give rise to early abrasion of the endless belt. In addition, the above abrupt stopping is performed using a braking device in order to prevent coasting rotation of the cutting blade. When the abrupt stopping was repeated, the above slippage led to very early abrasion of the endless belt, which necessitated immediate exchange of the endless belt.
Further, in the desk-top cutting machine, the workpiece is cut in such a way that the cutting blade is pressed down manually. Therefore, there is a case where the rotary speed of the cutting blade abruptly changes depending on the degree of pressing-down of the cutting blade by an operator. The abrupt change in the rotary speed produces vibrations in the cutting blade. This leads to level differences of cutting amount in the cutting surface 32a of the workpiece as shown in FIG. 9, generally called "saw marks" 33, which results in poor finishing of the cutting blade. For this reason, the operator must slowly cut the workpiece taking into consideration the pressing-down degree of the cutting blade so that the rotary speed of the cutting blade does not change abruptly.
Moreover, the rotary speed is relatively high during the no-load rotation of the cutting blade since it is set for the speed suitable to cutting. This leads to loud noise due to the cutting blade.
Further, when the cutting blade to be used is disk-shaped, inequality in the degree of flatness of the disk, vibrations of a machine body, etc. produce the resonance due to plane swing of the cutting blade between the real load rotary speed during cutting of the workpiece and the no-load rotary speed before making an incision into the workpiece. When the cutting operation is performed in this state, saw marks are produced on the cutting surface.
In view of the problems described above, in order to prevent slippage of the endless belt, it is necessary to control the rotary speed of the motor so that it does not exceed the permissible transmission torque of the endless belt with the result that the rotary speed of the cutting blade is adjusted to an appropriate rotary speed.
A conventional rotation control for the motor is disclosed, for example, in Japanese Patent Unexamined Publications No. Sho 61-10991 and No. Sho 61-180593. The former discloses that the rpm of the motor and the maximum torque are separately adjustable whereas the latter discloses that the motor control is carried out in such a way that in a motor capable of making reversible rotation, a regulating device is provided which detects the rpm and load current of the motor to fix the rpm and also regulate the motor load current so that the rpm to be fixed can be regulated. Such motor rotation controls disclosed in the publications are used in a finishing machine such as a power tool, and limited to only the load in machining.
It is well known to use an endless belt as means for transmitting motor driving force. However, no proposal has been made which can control the motor, over the entire process from motor starting to cutting and stopping, to the torque and rpm corresponding to the permissible transmission torque and the rpm preventing the resonance of a cutting blade.